It is known in the art to use heat treatable aluminum alloys in a number of applications involving relatively high strength such as aircraft fuselages, vehicular members and other applications. Aluminum Association alloys (“AA”)2×24, such as AA2024, AA2324 and AA2524 are well known heat treatable aluminum alloys which have useful strength and toughness properties in T3, T39 and T351 tempers.
The design of a commercial aircraft requires various properties for different types of structures on the aircraft. Especially for fuselage skin or lower wing skin it is necessary to have properties such as good resistance to crack propagation either in the form of fracture toughness or fatigue crack growth. At the same time the strength of the alloy should not be reduced. A rolled alloy product either used as a sheet or as a plate with an improved damage tolerance will improve the safety of the passengers, will reduce the weight of the aircraft and thereby improve the fuel economy which translates to a longer flight range, lower costs and less frequent maintenance intervals.
It is known in the art to have AA2×24 alloy compositions with the following broad compositional range, in weight percent:
Cu:3.7-4.4Mg:1.2-1.8Mn:0.15-0.9 Cr:0.05-0.10Si:≦0.50Fe:≦0.50Zn:≦0.25Ti:≦0.15                the balance aluminum and incidental impurities. Over time narrower windows have been developed within the broad 2024-series alloy range, in particular concerning lower combined Si and Fe ranges to improve on specific engineering properties.        
U.S. Pat. No. 5,593,516 discloses a high damage tolerant Al—Cu alloy with a balanced chemistry comprising essentially the following composition (in weight %):
Cu:2.5-5.5Mg:0.1-2.3Cumax−0.91 Mg + 5.59Cumin−0.91 Mg + 4.59Zr:up to 0.2, orMn:up to 0.8                balance aluminum and unavoidable impurities. It also discloses T6 and T8 tempers of such alloys which gives high strength to a rolled product made of such alloy.        
U.S. Pat. No. 5,897,720 discloses a high damage tolerant Al—Cu alloy with a “2024”-chemistry comprising essentially the following composition (in weight %):
Cu:3.8-4.9Mg:1.2-1.8Mn:0.3-0.9Si:<0.30, preferably <0.12Fe:<0.30, preferably <0.08Ti:<0.15, preferably <0.06                the balance aluminum and unavoidable impurities wherein the alloy is annealed after hot rolling at a temperature at which the intermetallics do not substantially dissolve. The annealing temperature is between 398° C. and 455° C.        
JP-A-07252574 discloses a method of manufacturing an Al—Cu—Mg alloy comprising the steps of hot rolling after continuous casting and specifying the cooling rate at the time of solidification. In order to benefit from the high cooling rates in the continuous casting operation the contents of Fe and Si are controlled such that the sum of Fe+Si exceeds as least 0.4 wt. %.
U.S. Pat. No. 5,938,867 discloses a high damage tolerant Al—Cu alloy with a “2024”-chemistry comprising essentially the following composition (in weight %):
Cu:3.8-4.9Mg:1.2-1.8Mn:0.3-0.9                balance aluminum and unavoidable impurities wherein the ingot is inter-annealed after hot rolling with an anneal temperature of between 385° C. and 468° C.        
EP-0473122, as well as U.S. Pat. No. 5,213,639, disclose an aluminum base alloy comprising essentially the following composition (in weight %):
Cu:3.8-4.5, preferably 4.0-4.5Mg:1.2-1.8, preferably 1.2-1.5Mn:0.3-0.9, preferably 0.4-0.7Fe:≦0.12, preferably max. 0.1Si:≦0.10                the remainder aluminum, incidental elements and impurities, wherein such aluminum base is hot rolled, heated to above 910° F. to dissolve soluble constituents, and again hot rolled, thereby obtaining good combinations of strength together with high fracture toughness and a low fatigue crack growth rate. More specifically, U.S. Pat. No. 5,213,639 discloses a required inter-anneal treatment after hot rolling the cast ingot within a temperature range of 479° C. to 524° C. and again hot rolling the inter-annealed alloy wherein the alloy may contain optionally one or more elements from the group consisting of:        
Cr:0.02-0.40 V:0.01-0.5 Hf:0.01-0.40 Cr:0.01-0.20 Ag:0.01-1.00 Sc:0.01-0.50.
Such alloy appear to show a 5% improvement over the above mentioned conventional 2024-alloy in T-L fracture toughness and an improved fatigue crack growth resistance at certain ΔK-levels.
EP-1170394-A2 discloses an aluminum sheet product with improved fatigue crack growth resistance having an anisotropic microstructure defined by grains having an average length to width aspect ratio of greater than about 4 and comprising essentially the following composition (in weight %):
Cu:3.5-4.5Mg:0.6-1.6Mn:0.3-0.7Zr:0.08-0.13                the remainder substantially aluminum, incidental elements and impurities. The examples show a Si-level in the range of 0.02 to 0.04 while maintaining a Cu-level of more than 3.0. Furthermore it is disclosed an Al—Mg—Si alloy (AA6xxx series) with Si levels between 0.10 and 2.50 but Cu levels below 2.0 and an Al—Mg alloy (AA5xxx series) with Si levels of up to 0.50 but Cu levels below 1.5. The first mentioned alloy has an improvement in compressive yield strength property achieved by respective sheet products in comparison with conventional 2524-sheet products. Furthermore, the strength and toughness combinations of such sheet products with high Mn variants have been described to be better than those of 2524-T3. Throughout the high anisotropy in grain structure the fatigue crack growth resistance could be improved.        
Furthermore, it is described that low copper-high manganese samples exhibited higher properties than high copper-low manganese samples. Results from tensile strength measurements showed that high manganese variants exhibited higher strength values than the low manganese variants. The strengthening effect of manganese was reported to be surprisingly higher than that of copper.